These days, optical amplifiers using rare earth element-doped optical fibers have been researched and developed in the fields of optical communication, optical signal processing, optical measurement, and so on. The rare earth element-doped optical fiber comprises a core having a refractive index higher than that of a cladding layer, and containing a doped rare earth element such as Er, Nd, Pr, etc., and the optical amplifier using the optical fiber comprises a light source for emitting an excitation light which is absorbed in the optical fiber inherently to the doped rare earth element to amplify a signal light transmitted through the optical fiber. Among optical amplifiers using several rare earth elements, an optical amplifier doped with Er has been remarkably enhanced in performance to provide a gain of more than 40 dB.
Conventional rare earth element-doped multiple-core optical fibers have been proposed firstly in the world by the inventors, and power amplifiers using the optical fibers have been proposed to provide high gains as shown in the Japanese Patent Kokai Nos. 5-299733 and 6-37385. In accordance with the use of such high gain-optical fiber amplifiers, signal lights of more than ten channels which are wavelength-multiplexed have been studied to be transmitted via optical fiber transmission lines. In such optical fiber transmission lines, the high gain optical amplifiers are required to have wide wavelength band characteristics in which signal lights having wavelengths ranging in a wide band are uniformly amplified.
In order to realize such wide band optical fiber amplifiers, several proposals have been studied as explained below.
The first proposal is that the wide wavelength characteristics are mainly enhanced, while the increase of gains is sacrificed, because a wavelength band is narrowered, when the maximum gain is obtained. In this proposal, the gain is suppressed by lowering the level of an excitation light, while the level of a signal light is not lowered, but is maintained to be an ordinary value, so that an optical fiber amplifier having the wide wavelength characteristics is obtained.
The second proposal is that a high concentration, for instance, up to 3% of aluminum is doped into a core of an optical fiber to provide the wide wavelength band.
The third proposal is that a length of an optical fiber is designed to be sufficiently shorter than, for instance, one half of an optimum length for an optical fiber amplifier, so that the wide wavelength characteristics are obtained, while the gain is slightly sacrificed.
The fourth proposal is that optical fiber amplifiers are connected in series, and variable attenuators are inserted into each two adjacent optical fiber amplifiers, so that the gain is suppressed to provide the wide wavelength band characteristics.
The fifth proposal is that optical fiber amplifiers are connected in series, and waveguide type March-Zender optical filters are inserted into each two adjucent optical fiber amplifiers, so that gains are equalized at a predetermined wavelength band to provide the wide wavelength characteristics.
The sixth proposal is that powers of signal lights to be wavelength-multiplexed are controlled at each wavelength at a light source. Thus, the uniformity of the gains and band characteristics is obtained.
In the above described proposals, however, there are disadvantages as explained below.
In the first proposal, an optical fiber amplifier is only used under a situation where powers of a signal light and an excitation light are limited.
In the second proposal, the maximum concentration of aluminum is 3% in a core of an optical fiber amplifier due to the restriction in a fabrication process thereof. As a result, a gain is approximately 26 dB, and a wavelength band is approximately 25 nm at a gain of 3 dB. This is not sufficient for the practical use of optical fiber amplifiers.
In the third proposal, a high gain and wide wavelength band characteristics are not obtained simultaneously, because an optical fiber is short.
In the fourth and fifth proposals, the efficiency becomes low, because gains are largely suppressed.
In the sixth proposal, a driving circuit in the light source becomes complicated, and the powers must be controlled at each wavelength dependently on each optical fiber amplifier.
As discussed above, the wide wavelength charcteistics are not consistent with the gains in the conventional optical fiber amplifiers having single cores doped with a rare earth element.